Żaneta Anna Mierzejewska, Zbigniew Oksiuta
Failure Analysis of a Femoral Hip Stem Made of Stainless Steel after Short Time of Exposure
FAILURE ANALYSIS OF A FEMORAL HIP STEM MADE OF STAINLESS STEEL
AFTER SHORT TIME OF EXPOSURE
Żaneta Anna MIERZEJEWSKA*, Zbigniew OKSIUTA*
*Faculty
of Mechanical Engineering, Division of Materials and Biomedical Engineering, Bialystok University of Technology,
ul. Wiejska 45c, 15-351 Białystok, Poland
[email protected], [email protected]
Abstract: This paper describes a case study summarising the failure analysis of a stainless steel femoral stem, which failed prematurely
within 36 months of exploitation in human body. In order to determine the mechanism of failure, a broken stem component were analyzed
by means of macroscopic and microscopic obserwations and hardness measurements. Metallurgical obserwations revealed that the tested
material does fulfill ASTM requirements. Scanning electron microscopy images revealed the presence of stress-induced cracking. The results
of the hardness revealed significant nonuniformity from the surface towards the inner part of the stem. It is assumed that any discontinuity
or defect on the fracture surface of the stem acted as preferential site for a crack nucleation and propagation by fatigue until the cross section
of stem was not able to sustain a load generated by a patient.
Key words: Hip Replacement, Endoprosthesis Stem Fracture, Isolated Fracture, Total Hip Arthroplasty Complications, Fatigue Fracture
1. INTRODUCTION
Human locomotor system is composed of interrelated ligaments by means of joints and muscles, which are controlled by the
nervous system (Dalaunay et al.,1996). A properly built and functioning hip joint allows for a smooth range of motion in multiple
planes. Any disease in the joints may lead to their harm, causing its
deformation, pain and loss of functionality. Total hip arthroplasty
is one of the most successful practices in orthopedics, which restores the correct function of the hip joint disrupted by fracture
or disease (rheumatoid arthritis, hip dysplasia, avascular necrosis
of the femur) (Scheerlinck et al., 2004; Griza et al., 2013). This procedure intends to remove damaged articular joints and replace
them with an artificial hip joint. During service life, artificial implants
are exposed to an aggressive environment such as corrosion, wear
and severe loading (Kamachi et al., 2003). These factors results
in a wide range of failure mechanisms. One of the most serious
complication is isolated stem fracture, consisting in breaking the implant inside the intramedullary canal, without damaging surrounding bone tissue. An incidence of this complication varies with many
factors, including the patient population, for example the body mass
index (BMI), physical activity, bone sideases or age and the geometry of the implant as well as kind of materials from which the artificial joint is made (Galante et al., 1980; Martens et al., 1974).
Biomaterials belong to the group of modern materials which are
used for the replacement and reconstruction of human body. A very
important group of biomaterials are metallic alloys, e.g.: stainless
steels, Co-Cr alloys and Ti alloys. One of the common biomaterial
used for bone implants and surgical instruments are austenitic
stainless steels. Good mechanical properties, adequate biocompatibility and relatively low price of those alloys make them very suitable material for biomedical applications. The most popular stainless
steels used for medical applications are 316L (ASTM A276-98b)
and Rex 734 (ASTM F1586) grade.
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An isulated failure of the femoral component after total hip arthroplasty is not a common occurrence from a statistical point
of view, however there are some cases of stems fractures reported
in the literature (Charnley, 1975; Mollan et al., 1985; Wilson et al.,
1992; Malchau et al., 2002; Lakstein et al., 2011).
Roffey et al. (2012) described an isolated fatige fracture of the
stem made of austenitic stainless steel after relatively short time
of implantation  4.5 years. The microstructure, hardness
and chemical analysis of the fractured stem revealed good agreement with ISO standard requirements for such kind of implants.
No defects such as cracks, non-metallic inclusions or corrosion
were detected during observations of the fractured area. Also, the
microstructure of the stem was homogeneous, with fine grains, typical for wrougth austenitic stainless steel. Detailed analysis of the
fracture surface revealed two points of fracture initiation located on
opposite sides of the stem. The location of these points, and the
direction of the propagation of fatigue striations suggested that the
stem fracture was caused by torsional loads.
Jarvi et al. (2007) also described an interesting case of a quadruple fracture of stem made of austenitic stainless steel, 316L
grade, after 4 years from imlpantation. Around the stainless steel
stem granulation tissue was observed, which according to the authors knowladge could be due to wear products of acetabular cup.
Striations on the fracture surfaces of the stem were observed as
the result of a fatigue failure.
The most curious case of double isolated stem fracture, also
made of austenitic stainless steel after 4.5 years of implantation,
was described by Sen et al. (2009). On the basis of observations
of the fracture area the authors proposed a hypothesis that the
damage was caused by cyclic-variable load, leading to fatigue fracture.
Hernandez­Rodriguez et al. (2010) observed failure mechanism of the 316LV stainless steel stem where a main crack was
originated on the anterolateral corner section of the material. Note
that most of the cases reported in the literature concern the stem
acta mechanica et automatica, vol.8 no.3 (2014), DOI 10.2478/ama-2014-0026
fracture made of 316 LV stainless steel grade, and there is no available information about fracture of the other kind of austenitic stanless steel, e.g. REX 734.
The main goal of this work is to analyse and describe a failure
mechanism of a hip stem made of austenitic stainless steel, REX
734 grade, after relatively short time of implantation, less than
3 years.
2. EXPERIMENTAL PROCEDURE
General image of the fractured stem is shown in Fig. 1. After
revision, two parts of the fractured stems were prepared to further
analysis. A small piece of material was cut from the stem and prepared for metallographic observations according to ASTM E130611 standard. The chemical composition of the stainless steel was
examined by means of a spark emission spectroscopy (Thermo
ARL Quantris, Switzerland). Microstructure observations after etching by means of the Vilella’s echant (picric acid 1 g; HCl 5 ml; ethanol 100 ml) were carried out by an optical microscope (OM) a Nikon Eclipse LV1000. The fracture surfaces of stem were examined
by means of Scanning Electron Microscopy (SEM) Hitachi S3000N, equipped with Energy Dispersive Spectroscopy (EDS).
Hardness measurements were performed using Vickers universal
indenter with a load of 5 kg. Grain size and hardness measurements were performed according to ASTM standards E112 and
E384, accordingly.
150 [mm]
Calculated Cr- and Ni- equivalents exhibited 12.80%
and 22.75%, respectively. Therefore, fully austenitic structure is expected. This was confirmed by the XRD measurements
of the tested stem (not presented here).
Tab. 1. Chemical composition of the stems and ASTM F1586 standard
(% weight)
ASTM F
1586
Sample
C
Si
Mn
P
S
0.08a
0.75
2-4.25
0.025 a
0.01 a
0.27
Mo
4.1
Cu
0.002
Ni
2-3
0.25 a
9-11
2.33
0.036
7.05
0.015
N
0.250.5
-
0.036
Cr
ASTM F
19.51586
22
Sample
21.14
(a – maximum content)
Nb
0.250.8
0.39
SEM-EDS linear analysis was also performed to observe
changes in the concentrations of main alloying elements along
a cross-section of the stem as it is presented in Fig. 2a. The results
presented in Fig. 2b confirmed a homogeneous distribution of alloying elements in the steel matrix.
a)
b)
Fig. 1. General view of the fractured stainless steel stem
3. RESULTS AND DISCUSSION
The results of chemical analysis of the tested steel, presented
in Tab. 1, revealed that the material of stem, in general, fulfils the
requirements of ASTM F1586. However, there are some differences in chemical composition of the tested stem in comparison
with the ASTM F1586 standard. Sulphur content is slightly higher
than specified, whereas the nickel content is slightly reduced. Note
that nitrogen, an element that stabilise a Fe- phase was not detected in the sample. It is well known that in austenitic stainless
steels chrome and nickel concentration and their equivalent elements content play very important role for stabilisation of the FCC
phase structure. Chromium and nickel equivalents can be calculated according to Equations (1) and (2).
RCr = (%Cr) + (%Mo) + 1.5(%Si) + 0.5(%Nb) + 2(%Ti)
(1)
RNi = (%Ni) + 30(%C + %N) + 0.5(%Mn)
(2)
Fig. 2. SEM-EDS linear analysis of the elements concentration
on the surface of the stem – (a) macroscopic view
(b) – elements distribution
OM image of the etched microstructure of the tested sample,
presented in Fig. 3 showed equiaxed austenitic grains and twins
characteristic for forged and annealed austenitic stainless steels.
Note, that no delta ferrite was observed.
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Żaneta Anna Mierzejewska, Zbigniew Oksiuta
Failure Analysis of a Femoral Hip Stem Made of Stainless Steel after Short Time of Exposure
In general, the microstructure is quite homogeneous,
with an average grain size of 363 m (no 7 according to ASTM).
However, larger grains near the surface of the stem were also observed which may cause an origin of a fatigue crack initiation of the
steel (Fig. 3b). This inhomogeneity of the grain size is difficult to
explain, however, this may be related to hardness results presented
in the section 3.3 (Fig. 6).
or line was created when the crack stops after short distance propagation. These patterns are not clearly visible, probably because
the fracture surfaces were severally smashed, as a result of friction
of both broken parts during prolonged walking after fracture. There
is another possible explanation reported by Brown (2014) and related to very fine microstructure of steel or unchanged load during
fatigue damage.
a)
b)
Fig. 4. Fatigue surface with fracture view showing the zones:
a) an origin, b) progression lines
c) and a final fracture zone
Fig. 3. Typical OM microstructure of the femoral stem;
(a) taken from inside of the stem
(b) taken from an edge of the stem
Fig. 4 shows a general view of the fractured surface
of the tested material. The image shows a several distinctive features typical for fatigue fracture: an origin where a crack can be
initiated (zone a), fatigue striations (zone b), and a final fracture
area (zone c). The fatigue fracture probably was initiated close to
the lateral edge of the material, and further propagated until a cross
section of the stem was no longer strong enough to bearing the
applied load and it finally broke. The fatigue zone is unique to fatigue fractures because it is the region where the crack grows from
the origin to the final fracture zone. It should be pointed out that the
geometry of the fractured stem did not reveal any changes.
No bending of stems was observed. Also, no traces of corrosion
were discovered.
Fig. 5 shows SEM images of the fractured surface. Fig. 5a presents the magnified cracks initiation region whereas Fig. 5b shows
some of the "beach marks" (progression lines) area. Each mark
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As the cracked stem continues to be stressed, the crack progresses across the section. The amount of material carrying the
load is decreasing, so the stress is obviously increasing. Fig. 5c
confirms extensive friction islands at the fracture surface, however
higher magnification image in Fig. 5d shows typical fatigue striations. In Fig. 5e numbers of secondary cracks were observed, propagated perpendicular to the loading direction. The length of these
cracks corresponds well to the grain size of the steel. This type
of fracture may be caused by the presence of non metallic inclusions. Fig. 5f shows the SEM image of the final fracture stage,
where the unbroken area is reduced to the point that the applied
stress exceeds the ultimate strength of the material.
The origin of fatigue fracture commonly takes place in the anterolateral site of the hip stem due to the highest stresses created
under in-service conditions.
The hardness measurements of both parts of the broken stem
was performed few milimeters below and above of the fracture.
An average hardness of tested material is ~ 335 HV/5 and fulfils
ASTM requirements. However, an interesting effect was observed,
not equal and strong dependence of hardness of the measurement
site. In the middle of the cross section of the stem the hardness
is 360 ± 2 HV/5 and close to the edge of the cross section of the
specimen is about 10% lower, 325±5 HV/5 (see Fig. 6).
acta mechanica et automatica, vol.8 no.3 (2014), DOI 10.2478/ama-2014-0026
a)
b)
c)
d)
e)
f)
Fig. 5. SEM images of the fracture surfaces of the tested steel failed by fatigue fracture demonstrating: (a) origin of the failure (b) beach marks area;
(c ) wear islands on the fracture surface ; (d) fatigue striation; (e) secondary cracks and fatigue tears; (f) final fracture area
Fig. 6. Hardness measurements of the tested stem
as a function of the distance from the edge of the sample
It is not a significant scutter of the hardness of the tested
specimen, however it is difficult to explani, because these results
are not consistent with the microstructure observations presented
in Fig. 3. According to the Hall-Petch equation (Petch, 1953)
with decreasing the grain size the ductility and hardness
of the material increase. On the other hand this statemant is not
valid for some groups of materials, especially with subgrain
microstructure.
Stem fracture in the femoral component of total hip arthroplasty
may have a complex etiology. The most important factors that
cause fracture of metallic stem should be mentioned: the hidden
material defects, incorrectly realized thermo-mechanical and /or
heat treatment, improper design of the implant (incorrectly matched
to the load), improper fixing in the bone or inapropriate exploitation
(Galante et al., 1980; Kishida et al., 2012). The fracture of femoral
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Żaneta Anna Mierzejewska, Zbigniew Oksiuta
Failure Analysis of a Femoral Hip Stem Made of Stainless Steel after Short Time of Exposure
stem may be also caused by one factor or the combination
of factors including: high stress applied in the stem due to high level
of activity or fast patient weight gain, undersized prosthesis, poor
proximal bone support or fixation, varus orientation of the stem and
material defects.
Series of tests conducted in this work revealed that the REX
734 stainles steel in general fulfills ASTM F1586 standard
requirements. The microstructure of the stem is homogeneous and
possesses relatively small grains. There are no changes caused by
intergranular or pitting corrosion, since homogeneous distribution
of main alloying elements, especially chromium, were detected.
On the contrary the results of the hardness revealed
measurable inhomogeneity coming from the edge towards innert
part of the material. This is difficult to explain and more tests have
to be conducted.
Macroscopic fatigue striations, commonly known as beach
marks, were visible. The beach marks were closely spaced
indicating a large number of cycles to failure, which eliminates the
possibility of sudden overload failure. Final failure occurred on the
lateral side corner. SEM examination of the fractured surface
revealed secondary cracks, which are the result of tensile and
compressive stresses induced in the material during patient’s
walking.
4. CONCLUSION
The main goal of this work was to analyse and describe a failure
mechanism of cemented hip stem made of austenitic stainless
steel, REX 734 grade. The tested material, in general, fulfils ISO
5832-1 standard, however, there are some factors such as: lower
Ni and N content, inhomogenous grain size, higher hardness
measured in the inner part of the stem and secoundary cracks
initiated at the non-metalic inclusions, that can explain early
fracture of the hip endoprosthesis after 3 years of implantation.
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Acknowledgements: This scientific work was supported by the Faculty
of Mechanical Engineering, Bialystok University of Technology, project
No MB/WM/14/2014.
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